A combustion chamber is generally annular in shape, and comprises a jacket providing axial openings upstream for letting air through and means of fuel distribution in the chamber enclosure. Close to such means, the jacket exhibits on its peripheral wall radial openings for letting through fuel sparking plugs. The plugs are fixed to the wall by force of spark plug guides which are globally cylindrical. These spark plug guides are welded by their front edge along the edge of an orifice provided in the wall of the jacket.
Currently the TIG welding method is performed manually. FIG. 1 represents the position of both parts. The spark plug guide tube 1 is engaged in the machined orifice 3 through the wall 5 of the combustion chamber before welding. The wall of the chamber, represented in axial section, exhibits a curve also in this axial direction. The edge of the orifice 3 is chamfered so as to provide a space with the external edge of the tube. Welding consists in placing a tungsten electrode T torch 7 above said space while bringing the metal by means of a rod B. During a first pass, a first bead is deposited at the bottom of the space for deeper penetration of the welding. In a second pass, the whole space is filled up.
This manual mode exhibits the shortcoming of requiring high dexterity from the operator by reason of the complexity of the junction zone. Besides, progressing along the welding bead, he must adapt permanently the quantity of energy to bring since the thicknesses and masses to be welded are not constant. Welding is not even. There ensue deformations requiring in particular final heating of the assembly to relax the internal stresses occurred in particular during the welding operation. Surface irregularities, inside the chamber or in the guide tube, involve moreover re-machining of the welding bead. Sometimes also, the stresses are high enough to generate cracks in the junction zone.
It should be reminded that Tungsten Inert Gas—TIG welding is an electric arc welding method with refractory electrode under gaseous atmosphere. This technique is used with or without any filler metal. Inert gas, generally argon—or helium-based, separates from the air the molten metal, the hot zones and the tungsten electrode. Any oxidisation is thereby prevented. In an inert gas stream, an electric arc is caused to flash between the infusible tungsten electrode and the part to be welded. The heat generated by the arc melts the edges of the part and any filler metal contributing to the formation of the welding bead. TIG welding is a manual or automated method. It is reserved for small and medium thicknesses, less than 5–6 mm, since the welding speeds for greater thicknesses are less than those of the other methods.